Since there is no air on the moon, there is the absence of a medium in which the cracking stones would set vibrations. The vibrations of the stones can not propagate in absence of a medium. So the sound of the stones cracking behind cannot be heard. In fact, no sound is produced. But this answer is on the assumption that our ears are open to the environment like on the earth which is not correct. In the absence of the air, there is no atmospheric pressure of which we are accustomed to. It will damage our body and ears. So we need space suits which isolates the whole body from the surrounding.
The sound of own footsteps can be heard because the vibrations produced can travel through our body to the ears.
